Executive functioning (EF) refers to self-regulation of thought and behavior. It has been implicated in several areas of cognitive and social development and deficits in EF are found in childhood disorders of autism and ADHD. Surprisingly little is known, however, about the normative course of EF development and the factors that guide it. Scientific inquiry and clinical practice pertaining to EF have been hampered by a lack of universal and appropriate measures of EF for preschool-age children. Existing measures suffer from a lack of generalizability, lengthy and non-child-friendly protocols, floor and ceiling effects, and ill-defined task demands. The proposed research has three major aims. First, new measures will be developed and tested with a diverse population of children from 30 to 60 months of age. The measures are multi-dimensional, brief, and sequenced to be age-appropriate. They will be standardized and normed for use in multiple settings. Second, longitudinal studies will be carried out to verify that individual children's development conforms to the sequence outlined in the first phase. As well, this design will include assessment of factors that might contribute to individual differences in EF including language, symbolic play, and parental input. Lastly, building on this newly acquired knowledge of normative development, training studies will be conducted to test the effectiveness of direct instruction in the tasks and training transfer from other skills such as symbolic play. This research will form the basis for more sound intervention strategies to improve children's overall functioning.